The present invention relates to a process for preparing low-viscosity reaction products of polyisocyanates containing activated groups ethylenically unsaturated which react, with polymerization, on exposure to actinic radiation.
The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or else electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology. Coating compositions based on this principle are thus referred to as radiation- or actinically curing or curable systems.
Because of the environmental and economic requirements imposed on modern coating systems, that they should use as little organic solvents as possible, or none at all, for adjusting the viscosity, there is a desire to use coatings raw materials which are already of low viscosity. Known for this purpose for a long time have been polyisocyanates with an allophanate structure as are described, inter alia, in EP-A 0 682 012.
In industry these substances are prepared by reacting a monohydric or polyhydric alcohol with large amounts of excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). This is followed by removal of unreacted diisocyanate by means of distillation under reduced pressure. According to DE-A 198 60 041 this procedure can also be carried out with OH-functional compounds having activated double bonds, such as hydroxyalkyl acrylates, although difficulties occur in relation to the preparation of particularly low-monomer-content products. Since the distillation step has to take place at temperatures up to 135° C., in order to be able to lower the residual isocyanate content sufficiently (<0.5% by weight of residual monomer), it is possible for double bonds to react, with polymerization, under thermal initiation, even during the purification process, meaning that ideal products are no longer obtained.
The preparation of low-monomer-content, allophanate-containing, polyurethane-based, radiation-curing binders is described in EP-A 0 867 457 and U.S. Pat. No. 5,739,251. These binders, however, do not carry activated double bonds but instead carry unreactive allyl ether groups (structure R—O—CH2—CH═CH2). It is therefore necessary to add reactive diluents (low molecular weight esters of acrylic acid), which introduce the required UV reactivity.
There has also been no paucity of attempts to prepare allophanates indirectly, from isocyanate derivatives, other than urethanes and isocyanates. For instance, EP-A 0 825 211 describes a process for synthesizing allophanate structures from oxadiazinetriones, although no mention is made there of radiation-curing derivatives containing activated double bonds. Transposition to the particular circumstances of radiation-curing systems is described in WO 2004/033522.
Another route is the opening of uretdiones (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419, and also US-A 2003 0153713) to give allophanate structures, which have also been already successfully transposed to radiation-curing systems (WO 2005/092942).
Both routes require high-grade base materials as starting material and lead only to an allophanate product which is rich in by-products.
U.S. Pat. No. 5,777,024 describes the preparation of low-viscosity radiation-curing allophanates by reacting hydroxy-functional monomers which carry activated double bonds with isocyanate groups of allophanate-modified isocyanurate polyisocyanates.
The radicals attached via the allophanate groups are saturated, and so any possible higher functionality is foregone.
EP-B 694 531 describes a multi-stage process for preparing hydrophilicized allophanates containing radiation-curing groups. In that case, however, first an NCO- and acrylate-functional urethane is prepared, which is hydrophilicized and subsequently allophanatized following addition of a further NCO- and acrylate-functional urethane. As the process temperature for the allophanatization, temperatures of 100-110° C. are specified.
Finally, a process was presented in EP-A 1 645 582 that leads, starting from simple diisocyanates, through reaction with hydroxy-functional acrylates, without distillation of the products, to low-viscosity allophanates. A disadvantage of this process, however, is that a satisfactory reaction rate is achievable only with ammonium salts that are difficult to obtain. Additionally, the viscosities of the products described are not as low as, for example, the viscosities of the allophanates obtainable by the process described in EP-A 0 825 211.
Lower viscosities can be achieved in the case of the method described in EP 1 645 582 by employing suitable, basic zinc catalysis. In spite of this the viscosity remains in need of improvement.
It was therefore the object of the present invention to provide a process with which allophanates of lower viscosity—as described in EP-A 1 645 582—and crosslinkable by actinic radiation (radiation-curing allophanates) are provided.
Starting from EP-A 1 645 582 it has now been found that a further reduction in the viscosity can be achieved with radiation-curing allophanates of this kind if a particular mixture of hydroxyalkyl acrylates is used to synthesize such allophanates. EP-A 1 645 582, although also describing in principle the use of these hydroxyalkyl acrylates, did not describe the particular viscosity-lowering effect of certain mixtures on the viscosity, despite the fact that the application likewise deals with the problem of low-viscosity allophanates.
The invention accordingly provides a process for preparing radiation-curing allophanates having a residual monomer content of less than 0.5% by weight and an NCO content of less than 1% by weight, the process comprising:
1) preparing NCO-group-containing urethanes having radiation-curing groups by reacting:
                A) one or more compounds containing isocyanate groups,        B) a mixture containing at least 90 mol % of hydroxyethyl acrylate and hydroxypropyl acrylate, in which 10-45 mol %, of hydroxyethyl acrylate is included,        C) optionally further, non-radiation-curing compounds containing NCO-reactive groups        D) optionally in the presence of a catalyst; and2) reacting the NCO-group-containing urethanes simultaneously or subsequently, without further addition of compounds containing isocyanate groups, in the presence of:        E) an allophanatization catalyst and,        F) optionally, a tertiary amine;the ratio of NCO groups of the compounds from A) to the OH groups of the compounds from B) and, where used, C) being 1.45:1.0 to 1.1:1.0.        
The invention further provides a coating composition comprising                a) one or more of the radiation-curing allophanates obtainable by the process of the invention,        b) optionally one or more polyisocyanates containing free or blocked isocyanate groups, which are free from groups which react, with polymerization, with ethylenically unsaturated compounds on exposure to actinic radiation,        c) optionally other compounds, different from those of a), which contain groups which react, with polymerization, with ethylenically unsaturated compounds on exposure to actinic radiation, and optionally contain free or blocked NCO groups,        d) optionally one or more isocyanate-reactive compounds containing active hydrogen,        e) one or more initiators,        f) optionally solvents and        g) optionally auxiliaries and additives.        